Such a process is described in DE 39 37 290 A1. The ionization electrode is in a d.c. circuit in this reference and the evaluation of the ionization current is problematic.
In Patent Application No. DE 44 33 425 A1, an alternating voltage, to which a d.c. voltage component that depends on the current of the ionization electrode is superimposed, is applied to the ionization electrode to improve the evaluability of the current flowing over the ionization electrode. An ionization voltage, which is a sufficiently accurate reflection of the current flame temperature and of the air ratio lambda (gas-to-air ratio), is derived from this.
It is also known that the heat output of a gas blower burner of a gas heater can be regulated by means of an automatic control unit corresponding to the heat demand, wherein the automatic control unit controls the speed of the blower as a function of an output set point, which depends on a room temperature set point and a heater flow temperature and/or the heater return temperature and an outside temperature.
Another control device for a gas burner has been known from DE 195 02 901 C1. It is based on the fact that the intensity of the flames is subject to continuous variations, i.e., there is a flickering flame pattern. It is recognized that the amplitudes of these variations depend on the gas-to-air ratio (lambda value) of the combustion gas. A safety flame monitoring to switch off the gas in the case of flame failure is not mentioned.
Gas-burning devices have been known to have to meet stringent safety requirements. According to safety regulations (EN 298), the flame failure controller in gas-burning devices intended for continuous operation performs a self-testing at regular intervals during operation, at least once an hour. In gas-burning devices intended for intermittent operation, the gas burner must switch off at least once within 24 hours in order to check the function of the flame failure controller. It is not ruled out that a defect may develop in the flame failure controller during the operation of the burner, and, in addition, the flame goes out. The automatic firing unit cannot recognize this at first and it cannot send a gas switch-off signal, as a consequence of which unburned gas is discharged until the next self-testing of the flame failure controller or until the burner is switched off.
An ionization flame failure controller, in which a capacitor charged to an operating voltage is discharged by the ionization current, has been known from DE 43 09 454 A1. The function of the ionization flame failure controller can be tested during the operation by means of a test signal. The ionization electrode itself and its connection cable and, in the case of certain disturbances, the capacitor cannot be tested. The flames are monitored only indirectly. In addition, the flame failure controller is tested by the test signal during periodically recurring time periods only.